This invention relates generally to apparatus and method for industrial mixing and processing and particularly to horizontally arranged industrial mixers having horizontally disposed mixing shafts extending through the mixing chamber.
Processing of a large variety of consumer and industrial products, such as food, plastic, pharmaceutical and chemical products, for example, usually involves one or more mixing steps for mixing the various component materials of the products. Such mixing steps are oftentimes accompanied by the simultaneous introduction or removal of heat, such as for drying the material being mixed or cooling heated products while they are mixed. Product mixing is also often accompanied by granulation or chopping of the material forming the product. Often, for numerous products, the materials are mixed in a dry, powdered or granular form, and the mixing process is referred to generally as solids mixing.
For accomplishing such solids mixing, large-capacity industrial mixers are utilized, which are able to handle very large loads of material for efficient and cost-effective mixing. One type of mixer design, which is suitable for solids mixing and is able to effectively mix large loads of material, is a horizontal mixer. Horizontal mixers have an elongated mixing chamber which is disposed generally horizontally with respect to the ground surface on which the mixer rests. More specifically, horizontal mixers generally comprise an elongated cylindrical mixing chamber, and an elongated, horizontal mixing shaft which extends through the chamber and rotates. A plurality of mixing tools depend generally perpendicularly from the horizontal shaft and rotate around the inside of the chamber when the shaft is rotated. The mixing tools are configured and dimensioned as required for the mixing process to follow the cylindrical inside walls of the chamber for proper mixing of all of the material in the chamber.
In a conventional horizontal mixer, the elongated horizontal mixing shaft extends out of the mixing chamber at both ends of the chamber through appropriate openings in the chamber end walls or head walls. At one end of the shaft, referred to as the drive end, the shaft is operably coupled to a drive motor and gearing which rotates the shaft. At the drive end, the shaft is coupled through a bearing structure located between the drive motor and the chamber. The bearing structure provides support of the shaft drive end and also ensures smooth rotation. A separate seal structure is then located further in along the length of the shaft and interfaces with the opening in the end wall through which the drive end of the mixing shaft extends.
The other end of the mixing shaft, referred to as the stub end of the shaft, is not driven, but rather rotates with the drive shaft. The stub end of the mixer also includes a seal structure to seal the stub end of the shaft and the end wall opening through which it extends. The seal structures at the ends of the shaft isolate the mixing chamber environment from the outside environment and generally prevent the passage or leakage of material into and out of the mixing chamber. The seal structures used in such horizontal mixers are therefore important to ensure the integrity and purity of the material being mixed and are also necessary for preventing leaks and protecting the health of workers in the area of the mixers.
As may be appreciated, leakage between the horizontal mixing chamber and the outside environment and atmosphere during mixing is undesirable. For example, edible products such as pharmaceuticals and foods must not be contaminated with foreign materials which may leak into the mixing chamber at the shaft end openings. Grease or oil associated with the drive motor and/or the shaft bearings must also be kept out of the mixing chamber. Furthermore, it is also equally important to contain the mixed material in the chamber and to prevent it from migrating and leaking to the outside environment through the shaft end openings of the mixer. This is particularly so if the material being mixed is a harmful chemical which cannot be directly contacted by the skin or if the mixed material produces a harmful vapor which may be released through the shaft openings.
Still further, it may be necessary to maintain a pressure differential between the horizontal mixing chamber and the outside environment to achieve proper mixing. For example, some mixing procedures require elevated pressures within the chamber which may be compromised by a leak. Furthermore, under such circumstances, a leak will tend to force mixed material through openings in the mixing chamber, such as out through the shaft end seals. Other procedures require that a vacuum be drawn in the chamber which would also be compromised by a leak. Also, a leak would draw contaminants into the chamber through the shaft end openings and seals.
While the seal structures of existing horizontal mixers operate somewhat adequately for their intended purpose, they have several drawbacks. More specifically, the seal structures at the stub end of the mixing shaft can be of particular concern, due to their location in the mixer.
First, many existing seal structures, including the stub end seal structures, are expensive. Because the rotating drive and stub ends of the mixing shaft extend through the end walls of the mixer, the seal structures must be dynamic seals which can handle both rotation and translation of the shaft, while still maintaining the seal. The seal structures, for example, may include elaborate dynamic seals with braided packing elements that surround the rotating shaft or may include expensive mechanical seals. The packing elements of certain dynamic seals are constantly worn by the rotation and linear movement of the shaft and thus are prone to wear and leakage. Therefore, constant maintenance and replacement of the dynamic seals are necessary. Some such seal structures must be coupled to an air line for preventing migration or leakage of the mixed material out of the chamber or the leakage of contaminants into the chamber. Mechanical seals, on the other hand, have highly polished faces which spin against each other under pressure. Such mechanical seals require precise, and therefore expensive, machining and polishing for proper operation and are also subject to wear and leakage. The complicated and intricate seal structures conventionally used for horizontal mixers are therefore expensive, not only to manufacture, but also to maintain and replace.
Secondly, the stub end seal structure is particularly prone to failure and leakage because of its position in the mixer. Therefore, the stub end seal and bearing structures must be maintained and replaced more frequently than the drive end seal structures. More specifically, the stub end of the shaft not only rotates during use, but also translates linearly in a longitudinal direction along the longitudinal, horizontal axis of the shaft. Since the drive end of the shaft is somewhat fixed due to the drive motor and other associated components, the longitudinal translation of the shaft caused by expansion and contraction of the shaft occurs primarily at the stub end. The shaft expands and contracts in length due to temperature changes during the mixing process. Constant exposure of the shaft to the variations in temperature caused by the heating and cooling of the mixing chamber and the heat generated by the mixing process causes some expansion and linear translation. Furthermore, the shaft itself may be actually heated or cooled such as by introducing steam, water or oil into a cavity in the shaft. Still further, the end walls or end plates of the mixing chamber will also move in and out longitudinally with respect to the shaft due to the temperature variations of the mixing chamber itself. Therefore, the seal structure and the packing elements at the stub end of the mixing shaft are exposed not only to rotational wear but also to significant translational wear, thereby making the stub end seal structure particularly prone to failure and leakage where the stub end of the shaft extends through the end of the mixer.
Leakage at the stub end sealing structure is a particular problem, because once the seal fails, material passes directly into or out of the chamber. There is generally no additional structure adjacent the failed stub end seal structure to further prevent leakage. The bearing supporting the shaft is often spaced away from the stub end sealing structure and away from the end of the mixing chamber and thus does not provide any significant sealing properties. Furthermore, the existence of the separate bearing may prevent additional sealing structures from being utilized at the shaft stub end. As a result, the operation and the integrity of the stub end sealing structure is a predominant concern when using horizontal mixers.
The frequent maintenance required for conventional stub end seals further increases the cost of the mixing process. As mentioned above, existing stub end seal structures used with horizontal mixers are prone to leakage, and thus, require maintenance in the form of replacing the worn packing elements or other mechanical sealing elements to prevent leaks. During such maintenance, whether scheduled or unscheduled, the mixer cannot operate, thus reducing the efficiency of the mixing process and reducing the overall cost-effectiveness of the mixer.
Still further, bearing failure from leakage may also be a problem at the shaft stub end. The stub end seal structure generally acts in concert with an external bearing. The bearings used with conventional horizontal mixers are intricately designed and have balls, rollers or other moving components which are lubricated with free lubricants, such as grease or lubricating oil. When the stub end seal structure leaks, the mixed material may migrate to the bearing and be trapped in the various cavities containing the balls, rollers and other components. This contaminates the lubricants. As a result, the bearing may wear prematurely and be damaged, or the bearing may even lock up and hinder the rotation of the shaft. Additionally, the mixed material may be chemically reactive and may corrode the bearing. Bearing maintenance and replacement due to leakage further increases the cost of operating a horizontal mixture.
Shaft deflection is also a concern associated with currently available horizontal mixers. The rotating shafts of horizontal mixers are designed to handle a certain amount of stress and to only deflect a predetermined amount due to the sag in the shaft between its supported ends. With stress and deflection as a limiting criteria, the shafts are designed and sized in diameter to achieve the acceptable deflection. For example, a deflection of {fraction (1/16)} of an inch may be acceptable in one mixer design, and thus the diameter of the shaft is sized accordingly. As will be appreciated, conventional horizontal mixers which support the shaft ends (and particularly the stub end) at conventional spaced apart bearings will require relatively large diameter shafts, increasing the costs of the mixer. A larger diameter shaft also requires larger bearing and sealing components, thus further increasing the costs of the mixer. Accordingly, it is desirable to support the stub end of the shaft close to the end wall of the mixing chamber to reduce shaft length and its deflection. It is also desirable to interface a stub end seal structure with a thinner shaft.
Therefore, there is a need for an improved structure for both sealing and providing rotational support of the mixing shaft of a mixer. There is particularly a need for an improved structure for sealing, supporting and rotating the shaft stub end in a horizontal mixer.
It is an objective of the present invention to address the drawbacks of the prior art and to provide a mixer which is less expensive to fabricate and operate than currently available mixers.
It is another objective to prevent leakage of mixed materials from the mixing chamber to the atmosphere and to prevent the leakage of outside contaminants into the mixing chamber to thereby effect better product containment and process environment integrity.
It is another objective of the present invention to seal the stub end of the mixing shaft of a horizontal mixer, while still maintaining the desired rotational integrity of the shaft.
It is another objective of the invention to reduce the effect of lateral and rotational wear on the stub end sealing structure to reduce the seal failure associated with such wear and reduce the required maintenance for the sealing structure.
It is still another objective to reduce the cost and complexity of the sealing and bearing arrangement in a horizontal mixer at the stub end of the mixing shaft.
It is still another objective of the invention to utilize a small diameter shaft to decrease the cost of the mixer.
The present invention addresses these and other objectives and provides a mixer for mixing and processing materials which comprises a mixing chamber with an inner space for receiving material to be mixed. An elongated rotatable mixing shaft extends through the chamber inner space and has a stub end and a drive end positioned proximate opposing ends of the mixing chamber. In accordance with one aspect of the present invention, a non-rotatable stub shaft extends through an opening in the stub end of the mixing chamber. Particularly, the stub shaft extends through an opening in the stub end head wall. The non-rotatable stub shaft is coupled to the stub end head wall for linear translation therethrough, but the stub shaft is generally prevented from rotating in the end of the mixing chamber. Accordingly, a need for a dynamic seal at the stub end wall is eliminated by the invention. For rotation of the mixing shaft, the stub end of the mixing shaft is rotatably coupled to the non-rotatable stub shaft inside the mixing chamber. Therefore, the stub end of the mixing shaft is contained completely within the mixing chamber, and there is no rotational engagement of the mixing shaft with the stub end head wall.
For sealing the stub end head wall, a seal structure is coupled between the stub shaft and the end of the mixing chamber for sealing the chamber inner space from the environment outside of the mixing chamber. More particularly, in one embodiment, the seal is coupled to a head plate which is then coupled to the head wall of the mixing chamber. The seal structure is operable for moving with linear translation of the stub shaft for maintaining the seal during such translation. The seal structure thus may be a suitable static seal, such as a bellows seal, because the stub shaft does not rotate. A translation bearing is coupled to the stub shaft at the stub end of the chamber on the outside of the head plate and supports the stub shaft during translation. The bearing structure may also generally prevent rotation of the stub shaft. Alternatively, a separate structure might be utilized to prevent rotation of the stub shaft.
For smooth rotation of the mixing shaft, bearing structures are operably coupled between the mixing shaft and the stub shaft. To that end, a journal recess is formed in the stub end of the mixing shaft and a portion of the stub shaft is journaled within the journal recess with the bearing structures operably coupled between the mixing shaft and the stub shaft. A passage may be formed to extend through the stub shaft and couple to the bearing structures for delivering lubricant to the bearing structures.
A cap is coupled to the stub end of the mixing shaft to close the journal recess. The journal portion of the stub shaft extends through the cap for rotatably coupling with the mixing shaft. A dynamic seal is coupled between the cap and the stub shaft at the interface therebetween and is operable for sealing the interface during rotation of the mixing shaft. The dynamic seal may be an appropriate rotational dynamic seal such as a precision mechanical seal, or a lip seal.
The present invention provides a static, non-rotational seal at the stub end wall of the mixing chamber, such that the mixing chamber completely contains the stub end of the rotating mixing shaft. The present invention therefore provides better product containment and process environment integrity. The advantages of the invention are further illustrated by the Detailed Description of the invention set forth hereinbelow.